The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention relates to the reduction of post etch gasses or byproducts in the Front Opening Unified Pod (FOUP) of a semiconductor cluster tool.
Semiconductor process tools often post-etched wafers with pre-etched wafers in the Front Opening Unified Pod (FOUP). These wafers are not under vacuum, but are exposed to ambient conditions present in the wafer-fab. Typical main chamber processing chemistry involves the use of hydrogen bromide (HBr), chlorine (Cl2) and other gases. Bromides absorbed on the surface of the processed wafers tend to give off gas (outgas) inside the FOUP and condense as a brominated haze on the unetched wafer surface. These brominated condensates may become an etch defect due to micromasking and may result in yield reduction. A microwave post-etch treatment can eliminate brominated cross contamination caused by the outgas of bromide etched wafers. This hazardous material can also diffuse into surrounding areas and atmosphere, causing harm to operators.
A post process O2 flush can reduce the residual gasses on the wafer, however, adding the process step adversely affects wafer processing throughput by requiring additional time.
An alternative method to reduce wafer defect requires separation of processed wafers from pre-processed wafers. The approach can be achieved by adding an extra FOUP station, or locating wafers to buffer stations or other places inside the Transfer Mechanism (TM) to avoid cross contamination. However, adding an additional FOUP station increases the footprint of the tool, increasing the cost of ownership. Locating wafers to buffer stations or other places inside the TM has other issues as well. First, there are more wafers than spaces provided by the buffer or other stations. Second, relocating the wafer to alternate stations consumes time, and thus impacts wafer throughput. In addition the environmental issue of harmful gasses is not addressed by the above solutions.
One effective method practiced on 200 mm tools is to purge the FOUP with N2 or clean dry air sprayed from nozzles. Due to hardware limitations, to date, only fixed nozzles have been used and these are located outside of the FOUP but inside the TM.
Flow analysis shows that the position of these nozzles are one of the primary factors on the flow patterns through the FOUP. The back-flow from the FOUP should be minimized since the back-flow most likely contains purged air and possible contaminants, since it has already circulated through the FOUP. FIG. 1 shows flow patterns when purge nozzles are located just outside of the FOUP. FIG. 2 shows flow patterns when purge nozzles are positioned 2 inches inside the FOUP. FIG. 3 shows flow patterns when purge nozzles are positioned about 4.4 inches from the door inside the FOUP. Therefore, the further inside the FOUP the purge nozzles are positioned, the more desirable the flow patterns become from a purging perspective.
Another factor which affects flow patterns is the mass flow rate. FIG. 4 shows flow patterns when the flow rate is too high. Most of the mass flows along the FOUP wall and as a result, back flow concentrates at the center of the FOUP. When mass flow rate is optimal, mass flows in through the sidewall, purges through the wafers and then flows out the ATM, which is the collection site for wasted air.